1. Field of the Invention
This invention generally relates to an optical semiconductor device.
2. Description of the Related Art
Generally, a wavelength-tunable semiconductor laser has a gain for a laser emission and can select a wavelength of the laser light. There are some methods of selecting a wavelength. For example, the methods include a method of changing a resonant wavelength of loss or gain by changing a refractive index or angle of a diffractive grating or an etalon provided in a laser cavity. And the methods include a method of changing a resonant wavelength of the laser cavity by changing an optical length in the laser cavity (refractive index or a physical length of the laser cavity).
The method of changing the refractive index has an advantage in reliability or manufacturing cost, because a mechanical operating portion is not necessary being different from the method of changing the angle or length. The refractive index changing method includes changing a temperature of an optical waveguide, changing a carrier density in the optical waveguide by providing a current, and so on. A semiconductor laser having a Sampled Grating Distributed Reflector (SG-DR) is supposed as a wavelength tunable laser that changes a temperature of an optical waveguide, where the SG-DR has a wavelength selection function.
In this semiconductor laser, if a reflection spectrum of a plurality of SG-DR regions (reflection region) is controlled preferably, a predetermined wavelength can be selected with a vernier effect. That is, this semiconductor laser emits a laser light at a wavelength where reflection peaks of two SG-DR regions are overlapped with each other. It is therefore possible to control the lasing wavelength by controlling each of the reflection peaks of the SG-DR regions.
Generally, a heater is provided on a surface of the SG-DR region. The temperature of the optical waveguide of the SG-DR region is changed when the heater generates heat. As a result, the refractive index of the SG-DR region changes. It is therefore possible to control the reflection peak wavelength of the SG-DR region by controlling the heating value of the heater. The heater needs an electrical power when generating heat. And so, Japanese Patent Application Publication No. 9-92934 discloses a method of controlling a refractive index of a reflection region by providing an electrical power to a heater.
Here, a wavelength controllable range of the reflection peak is proportional to an amount of refractive index change of the optical waveguide, that is, an amount of temperature change of the optical waveguide. It is necessary to enlarge the voltage to be applied to the heater in order to enlarge the wavelength controllable range. However, it is preferable that the semiconductor laser is controlled with low electrical power, from a viewpoint of a drive circuit design. It is therefore difficult for the heater to generate sufficient heat which enlarges the wavelength controllable range of the semiconductor laser.
On the other hand, the heater may be divided into heaters, the heaters may be connected in parallel and the electrical resistance of the heater may be reduced. Generally, provided electrical power P has a relationship with a resistance R of the heater and the applied voltage V, and is shown as P=V2/R. And it is possible to enlarge the provided electrical power with the same voltage, when the electrical resistance R is reduced. However, in this case, a process of manufacturing a refined heater and an electrode coupled to the heater is complicated, and a cost is increased. In addition, if the number of the electrode is increased, a heat radiation through the electrode to outside of the optical waveguide is increased. Then, the amount of the temperature change, that is, the efficiency of the heater, is reduced even if the same electrical power is provided.